isometric game area problem

helloooo. i am totally new here and not strictly from strong maths background so bear with me.

im trying to make an isometric videogame, and there comes a point where i need to determine which of the game floor tiles the mouse is hovering over fomr the mouse x,y.

the problem is i need to determine the top left x,y value of a sprite when the mouse is over it, but only within the actual tile shape itself not the mask area of the sprite.

the dimensions are thus:
sprite height 32 pixels, width 64 pixels, the pixels numbers start at 0 rather than 1 from the top left of the sprite, and the actual shape of the tile is a diamond pointed at the middle two pixels of each horizontal and vertical face.